An optimized technique for reliability analysis of safety‐critical systems: A case study of nuclear power plant

Stochastic models are extensively used in quantifying the reliability of safety critical systems. These models use the state‐space model for reliability quantification. Markov chain is comprehensively used in describing a sequence of possible events of any system in which the probability of each event depends only on the state attained in the previous event. Markov chains are convenient to model the software system of the SCS with the help of Petri Nets, a directed bipartite graph widely used for the verification and validation of real‐time systems. However, the stochastic model suffers from the state‐space explosion problem. In this paper, we proposed a technique for reliability analysis of safety critical systems, excavating into the coherent optimization of Markov chain. The approach has been validated on 17 safety critical systems of nuclear power plants.     

Deterministic and probabilistic contingency selection

Abstract

Steady‐state contingency selection is performed on the Taiwan power system using both deterministic and probabilistic approaches. In the deterministic approach, both the real power performance index (PIp) and the voltage performance index (PIv) are evaluated to reach the desired ranking lists of line outages. Generator reactive power output limits (Q‐limits) are taken into account throughout the contingency selection procedure. To examine the effect of voltage control on contingency selection, both the case with a fixed tap changer position and the one with a variable tap changer position are considered. An important observation from this study is that the masking problem, which has been reported to take place in real power contingency selection, does not seem to appear in voltage contingency selection. Thus, it is recommended that in voltage contingency selection the summation in PIv be made over all buses while in real power contingency selection the summation in PIp be carried out only on overloaded lines. A new method for probabilistic contingency selection is also developed using the outage statistics of Taiwan power systems.     

An adaptive statistical method for power quality analysis

Digital fault recorders installed for monitoring current and/or voltage waveforms acquire and store vast amount of waveform data for post processing. Because of this, effective offline automated event detection from acquired data is necessary. In this work, we propose a new automatic event detection method which takes the acquired data and produces event flags at instances of events. The method is based on the statistical analysis of adaptive decomposition signals. The combination of an adaptive prediction filter-based subband decomposition structure with a rule-based histogram analysis block produced successful detection and localization results on our real-life power system transient data.     

A novel method for evaluating the secondary voltages of transformers in primary substations based on sensitivity coefficients

This paper provides a novel method to evaluate the secondary voltage of transformers in primary substations. The method is focused on the secondary voltage deviation which is a function of real power, reactive power, tap position, and primary voltage of the transformer in transmission systems. It helps dispatchers calculate and predict voltage deviation before introducing voltage regulation and reactive power compensation. The proposed approach is first to derive the formula of secondary voltage sensitivity coefficients from power flow equations. Real operational data are then substituted into the formula to get sensitivity coefficients. Finally, the corresponding secondary voltage is calculated with the sensitivity coefficients obtained. Implementing historical data of three substations in the above algorithm shows that the errors between calculated and actual voltages are less than 0.15%. The proposed method can not only accurately estimate the secondary voltage of the primary substation, but also be used to calculate the compensating reactive power as well as determine the tap position of the transformer, thereby improving the voltage and reactive power control strategies of power transmission systems.     

Event oriented system analysis

This article presents an attempt towards a probabilistic event oriented system analysis in engineering. Engineering systems are represented as either complete or incomplete systems of events and as compounds of various subsystems of events. The event oriented system analysis investigates important subsystems in engineering systems, such as operational modes and failure modes and their interrelations. The analysis is also applicable to engineering systems with various relations among the sets of events, such as mutually exclusive and inclusive sets. Further, the systems and subsystems are subjected to probability and uncertainty analysis. The system uncertainty analysis is based on entropy. General relations among the probability, uncertainty of the system and uncertainties of the subsystems are derived by using information theory. Specific mathematical aspects and available methods in the uncertainty modelling of systems and subsystems are summarised. Numerical examples confirm the relevance of the event oriented system analysis and indicate potential improvements in system design.     

A simple power supply and control unit for pulsed operation of a microchannel plate imaging detector

A simple power supply and control electronic system to operate microchannel plate (MCP) detectors for imaging transient events such as X-ray emission from laser-produced plasmas is described. The power supply consists of two parts, viz. a high voltage pulse generator of 1 kV amplitude with a variable pulse duration of 100 μs to 1 ms for the MCP input, and a regulated 5 kV d.c. for biasing the phosphor screen. The control unit synchronizes the high voltage pulse with the event and operates a safety switch to guard the detector against any accidental rise in the background pressure. A signal-to-noise ratio ≥ 75 is obtained, limited by the noise contribution of the Charge Coupled Device (CCD)-frame grabber combination attached to the MCP. The system is compact, has provision for both positive and negative pulses, is extendable to higher voltages for operating two-stage MCP, and is immune to electromagnetic interference.     

Optimal reactive power dispatch in a large power system with ac?dc and facts controllers

With the increased loading of existing power system, the problem of voltage stability and voltage collapse has become a major concern in power system planning and operation. The dependence of the system voltage profile on reactive power distribution forms the basis for reactive power optimisation. The technique attempts to utilises fully the reactive power sources in the system to improve the voltage profile and also to meet the reactive power requirements at the AC-DC terminals to facilitate the smooth operation of DC links. The method involves successive solution of steady-state power flows and optimisation of reactive power control variables with unified power flow controllers using linear programming technique. The proposed method has been tested on a real life equivalent 96-bus AC and a two terminal DC system.     

Design and control of single-phase dynamic voltage restorer

Dynamic voltage restorer (DVR) is a custom power device used in electrical distribution system for power quality improvement. It ensures regulated voltage supply to the sensitive loads, even in case of voltage sag and swell disturbances in the distribution network. It is a series connected device and compensates voltage sag and swell by injecting a voltage with the help of a series transformer. The injection of an appropriate voltage component in the event of a voltage disturbance requires a certain amount of real and reactive power. Conventionally, DVR consists of an energy storage device, which supplies the required power over the limited duration of the sags. Large magnitude and long duration of sags lead to heavy financial investment in energy storage unit. To overcome this limitation, a single-phase back-to-back converter-based DVR is implemented in this work, which eliminates energy storage requirement. The integration of series and shunt converter makes the DVR capable of bidirectional flow of energy. Therefore, the key advantage of this topology is its capability to compensate for long-term voltage sag and swell. Modelling of the DVR and its controller design is included in this paper. The effectiveness of control schemes, protection schemes and starting sequence of operation of DVR is verified through detailed simulation studies. A scaled down laboratory prototype of DVR is developed. The viability of these schemes is confirmed by the experimental results generated from the laboratory prototype. Various challenges faced during the prototype development and corresponding solutions are also discussed in this paper.     

Feasibility of online collaborative voltage stability control of power systems

The authors present the results of feasibility study of a novel concept of power system online collaborative voltage stability control. Online collaboration between power system controllers is proposed in order to enhance their overall performance and efficiency to cope with the increasing operational uncertainty of modern power systems. The framework of the proposed online collaborative voltage stability control is first presented, which is based on the deployment of multi-agent systems and real-time communication for online collaborative control. Then, two of the most important issues in implementing the proposed online collaborative voltage stability control are addressed: (1) Error-tolerant communication protocol for fast information exchange among multiple intelligent agents; (2) Deployment of multi-agent systems by using graph theory to implement power system post-emergency control. Results of testing the proposed online collaborative voltage stability control in the case of the 10-machine 39-node New England power system are presented. Results of a feasibility study by means of a simulation are given that take into consideration low-probability cascading faults in the power system.     

Risk implications of digital reactor protection system operating experience

This paper summarizes an in-depth review of the US nuclear operating experience with the first generation of digital reactor protection systems. The accumulated operating experience from 1984 to 2006 on these first generation digital reactor protection system functions exceeds 1.27 million hours (145.5 yr). A review of failure event reports identified 141 specific events associated with these systems on seven US nuclear power plants. Twenty-six of these events involved some type of common cause failure mechanism (predominantly redundant sensors/channels being out of calibration), which temporarily rendered redundant portions of the overall trip function degraded. Most of these failures were found not to be unique to digital systems. Six of the common cause failure events were more severe and involved situations where incorrect addressable constant data sets were systematically loaded into all redundant computer channels due to personnel errors. One of these events involved a latent software design change error introduced during a software update, which would prevent proper operation, given an unlikely event involving failure of three out of four sensors of one type.Based upon this review of digital system operating experience, a series of risk assessment calculations were performed to evaluate the safety significance of the observed failure events. From the insights gained in this work, it is possible to develop a framework for establishing digital reactor protection system reliability requirements that can be related back to regulatory safety goal objectives and operating experience.     

Event storm detection and identification in communication systems

Event storms are the manifestation of an important class of abnormal behaviors in communication systems. They occur when a large number of nodes throughout the system generate a set of events within a small period of time. It is essential for network management systems to detect every event storm and identify its cause, in order to prevent and repair potential system faults.This paper presents a set of techniques for the effective detection and identification of event storms in communication systems. First, we introduce a new algorithm to synchronize events to a single node in the system. Second, the system's event log is modeled as a normally distributed random process. This is achieved by using data analysis techniques to explore and then model the statistical behavior of the event log. Third, event storm detection is proposed using a simple test statistic combined with an exponential smoothing technique to overcome the non-stationary behavior of event logs. Fourth, the system is divided into non-overlapping regions to locate the main contributing regions of a storm. We show that this technique provides us with a method for event storm identification. Finally, experimental results from a commercially deployed multimedia communication system that uses these techniques demonstrate their effectiveness.     

COMPOUND DYNAMIC EVENT METAMODELS FOR ELECTRONIC ASSEMBLY LINE SYSTEMS

In the assembly line systems of the electronics industry, production controls based on steady-state conditions have proved ineffectual in coping with dynamic events, such as machine breakdowns, part supply shortages, and high priority job order processing, which can occur individually or simultaneously.

There are few tools that give results quickly as to how many assemblies will be delayed by a dynamic event and how long it will take to recover from the impact. Computer simulation is available, but its lengthy execution time has hindered its application in real time.

Previous research discovered that the patterns of these dynamic events can be represented by metamodels in the solution form of first order systems. In this paper, these results are unified and it is further shown that compound dynamic event metamodels can be developed from the individual metamodels using linear additivity. These metamodels are fit to the output from the simulation; and the resulting equations can be used in real time to measure the number of assemblies that are delayed due to the dynamic events.

To demonstrate the potential application of compound dynamic event metamodels for decision making in real-time production control, a Decision Support System (DSS) is described that contains the dynamic metamodels in its model base. With this program, the impact of the dynamic events on production can be obtained virtually instantaneously. A case study is presented to support this conclusion.     

Structure-preserved power-frequency slow dynamics simulation of interconnected ac/dc power systems with AGC consideration

A structure-preserved power-frequency slow dynamics simulation model is suggested for interconnected ac/dc power systems with automatic generation control (AGC) consideration, which will be applied to study relevant emergency control in future so that the bulk system viability crisis caused by load-frequency slow dynamics can be released. In the model, the network structure of interconnected power systems is entirely preserved, and the multi-area dynamic load flow (DLF) is developed for simulation. The generator speed governor and rotor dynamics, load-frequency characteristics, simplified models for high voltage direct current (HVDC) transmission and flexible ac transmission systems (FACTS) device thyristor controlled series capacitor (TCSC) suitable for long-term dynamics are considered with their AGC interfaces kept for future emergency-AGC study. However, at this stage, the sub-problem of reactive power and voltage is neglected for modelling simplicity and dc load flow is thus used for network solution. The concept of area centre of inertia (ACOI) is used based on the assumption of uniform frequency in each control area similar to that of the conventional single-area DLF calculation. The application of ACOI concept is attractive because the signal can be obtained from wide-area measurement systems (WAMSs) in real time and used to enhance long-term frequency stability through advanced control in future. The computer test results from 2-area 4-machine and IEEE 30-bus power systems demonstrated the validity and effectiveness of the suggested model and corresponding algorithm.     

Reactive power/voltage control in distribution systems under uncertain environment

A novel method for reactive power/voltage control in distribution systems under uncertain data environment is presented. This is achieved by minimising boundary real power loss or boundary total real power demand while satisfying constraints on boundary bus voltage magnitudes. Such an approach is more realistic and intuitively satisfying as compared with the crisp solution with deterministic data assumption. On-load tap changing facility of the transformer at the substation and the shunt capacitors at the substation and feeders are used as control variables. A truly nonlinear fuzzy distribution power flow is used, in which simultaneous non-statistical uncertainties in load forecast, load model coefficients and network parameters are incorporated. Genetic algorithm is used to solve the problem. In order to demonstrate the effectiveness of the proposed method, results for balanced 30-bus and unbalanced 25-bus and modified 123-bus systems have been obtained.     

New centralised adaptive load-shedding algorithms to mitigate power system blackouts

Conventional under-frequency load-shedding scheme is designed to retrieve the balance of generation and consumption following a disturbance. In the conventional load-shedding method, frequency settings, timedelay settings and the amount of load to be shed in each step are constant values. The loads to be shed by this scheme are also constant load feeders and not selected adaptively. This constant non-adaptive load-shedding algorithm is not the most efficient scheme for all power system disturbances. Application of centralised loadshedding algorithms could enhance adaptability of the load-shedding schemes. Two centralised adaptive loadshedding algorithms are proposed. The first algorithm is response-based and the second one is a combination of event-based and response-based methods. The proposed methods are capable of preserving power system instability even for large disturbances and combinational events. They use both frequency and voltage variables to select appropriate amount of load shedding. Parameters of the suggested schemes are also selected adaptively based on the magnitude of the disturbance. Performances of the proposed algorithms are evaluated by the application of the adaptive algorithms to the distributed and dynamic simulated model of a real power system. Obtained simulation results confirm that by using these algorithms various power system blackouts may be prevented.     

A systematic approach to analysing errors of commission from diagnosis failure in accident progression

The accident scenarios of a nuclear power plant are composed of an initiating event (IE), additional events/failures and human inappropriate actions, the combinations of which lead to irreversible consequences. In such a dynamic situation, operators should diagnose the occurring events/failures (including an initiating event and additional events) and assess the related situations utilising the available resources such as operating procedures or human–machine systems to control and maintain the plant in a stable condition. The misdiagnosis or diagnosis failure of the occurring events could cause critical human inappropriate actions that aggravate the plant condition, which is termed as errors of commission (EOCs). This paper presents a methodology for analysing the potential for diagnosis failure of the initiating and additional events and the consequent EOC events, based on the operating procedures, in the accident scenarios of nuclear power plants. The method to be presented categorizes the diagnostic situations in the accident scenarios into three cases according to the structure of the emergency operating procedures (EOPs) and the time of the occurring events: (1) the diagnosis of an initiating event, (2) the diagnosis of both an initiating event and an additional event when an additional event occurs prior to the performance of the diagnosis procedure, and (3) the diagnosis of an additional event when an additional events occurs after the performance of the diagnosis procedure. The application of the method is illustrated through three case example scenarios: (1) the power-operated relief valve (PORV) or the pressurizer safety valve (PSV) LOCA, (2) the loss of all feedwater (LOAF) event (loss of main feedwater*loss of auxiliary feedwater), (3) the sequence of<the station blackout (SBO)*loss of turbine-driven (or diesel-driven) auxiliary feedwater*PSV stuck-open*recovery of AC power>.     

Design of reactive power compensators for distribution systems with harmonic distortion

Abstract

The objective of this paper is to derive a systematic algorithm to decide the optimal location and size of shunt capacitors and filters for distribution systems with harmonic distortion. In this paper, the problem of reactive power compensation is first formulated as a nonlinear programming of minimization of real power loss and capacitor cost under voltage constraint to decide the optimal locations and sizes of shunt capacitors. The harmonic load flow is then applied to solve the total voltage harmonic distortion factor (HDF). Finally, the tuning frequency of the single‐tuned filter and the capacity and voltage ratings of the corresponding reactor and capacitor are determined so that both the harmonic distortion and the reactive power compensation can be solved simultaneously. To demonstrate the performance and effectiveness of the proposed methodology, a practical distribution feeder with nine large industrial customers is selected for computer simulation. It is concluded that proper design of harmonic filters should be undertaken to solve the harmonic resonance problem, as well as the reactive power compensation for distribution systems with nonlinear loads.     

Solid‐State Nanopore Detection of Protein Complexes: Applications in Healthcare and Protein Kinetics

Protein conjugation provides a unique look into many biological phenomena and has been used for decades for molecular recognition purposes. In this study, the use of solid‐state nanopores for the detection of gp120‐associated complexes are investigated. They exhibit monovalent and multivalent binding to anti‐gp120 antibody monomer and dimers. In order to investigate the feasibility of many practical applications related to nanopores, detection of specific protein complexes is attempted within a heterogeneous protein sample, and the role of voltage on complexed proteins is researched. It is found that the electric field within the pore can result in unbinding of a freely translocating protein complex within the transient event durations measured experimentally. The strong dependence of the unbinding time with voltage can be used to improve the detection capability of the nanopore system by adding an additional level of specificity that can be probed. These data provide a strong framework for future protein‐specific detection schemes, which are shown to be feasible in the realm of a ‘real‐world’ sample and an automated multidimensional method of detecting events.     

Coordinated stability control

An adaptive coordination scheme is proposed to improve the transient stability, voltage security and frequency damping of a power system network by optimally scheduling the parameters of the available controllers. A set of real-time stability indicators, including a transient stability index, transient voltage stability index and damping factor are used to reflect the stabilities of power systems. Then, the relative significances of a group of controllers to the power system are used as references for the optimiser to schedule new controller parameters. This coordinated control scheme effectively allocates power system resources to handle different stability issues in different time frames. The results suggest that power system security can be further enhanced by means of coordinated control.     

Methods and practices used in incident analysis in the Finnish nuclear power industry

According to the Finnish Nuclear Energy Act it is licensee's responsibility to ensure safe use of nuclear energy. Radiation and Nuclear Safety Authority (STUK) is the regulatory body responsible for the state supervision of the safe use of nuclear power in Finland. One essential prerequisite for the safe and reliable operation of nuclear power plants is that lessons are learned from the operational experience. It is utility's prime responsibility to assess the operational events and implement appropriate corrective actions. STUK controls licensees' operational experience feedback arrangements and implementation as part of its inspection activities. In addition to this in Finland, the regulatory body performs its own assessment of the operational experience. Review and investigation of operational events is a part of the regulatory oversight of operational safety. Review of operational events is done by STUK basically at three different levels. First step is to perform a general review of all operational events, transients and reactor scram reports, which the licensees submit for information to STUK. The second level activities are related to the clarification of events at site and entering of events' specific data into the event register database of STUK. This is done for events which meet the set criteria for the operator to submit a special report to STUK for approval. Safety significance of operational events is determined using probabilistic safety assessment (PSA) techniques. Risk significance of events and the number of safety significant events are followed by STUK indicators. The final step in operational event assessment performed by STUK is to assign STUK's own investigation team for events deemed to have special importance, especially when the licensee's organisation has not operated as planned. STUK launches its own detail investigation once a year on average. An analysis and evaluation of event investigation methods applied at STUK, and at the two Finnish nuclear power plant operators Teollisuuden Voima Oy (TVO) and Fortum Power and Heat Oy (Fortum) was carried out by the Technical Research Centre (VTT) on request of STUK at the end of 1990s. The study aimed at providing a broad overview and suggestions for improvement of the whole organisational framework to support event investigation practices at the regulatory body and at the utilities. The main objective of the research was to evaluate the adequacy and reliability of event investigation analysis methods and practices in the Finnish nuclear power industry and based on the results to further develop them. The results and suggestions of the research are reviewed in the paper and the corrective actions implemented in event investigation and operating experience procedures both at STUK and at utilities are discussed as well. STUK has developed its own procedure for the risk-informed analysis of nuclear power plant events. The PSA based event analysis method is used to assess the safety significance and importance measures associated with the unavailability of components and systems subject to Technical Specifications. The insights from recently performed PSA based analyses are also briefly discussed in the paper.